Bicycle used for track racing. Although specialised models (with low profile bars and bar extensions) are used for time trials and pursuiting, the generic track design is a standard diamond frame with a fairly high bottom bracket (for improved ground clearance when riding slowly on the bankings) and straight rear-facing dropouts for easy chain tension adjustment.
The equipment is stripped down to basics: single gear fixed wheel - direct drive - transmission, usually with a gear ratio around the 50 x 15 mark (depending on the race, the rider and the track geometry), and no brakes; the only means of deceleration is by stamping back on the pedals or being caught by someone standing beside the track. Although this may seem dangerous at first sight, in practice the ability to ride knowing that nobody in front of you is going to brake outweighs it. Some riders, particularly sprinters, still favour steel bars and stem and old-fashioned toeclips and straps - often with two pairs of straps - rather than clipless pedals. Cranks are usually 165 mm length rather than the 170-175 mm used on the road, helping both ground clearance and fast pedalling. No superfluous projecting parts of any sort are allowed (I was once stopped from riding a track meeting because my rear axle extended too far - about a quarter of an inch - beyond the tracknuts).
At present the use of disk wheels is permitted on the rear only for events where riders are in direct competition, front and rear for time trials and pursuits. Some of the more technically demanding tracks apply specific design restrictions, particularly as regards pedal clearance, since velodrome managers do not like having chunks gouged out of their perfectly aligned wooden boards.
A track bike can be made road legal in many jurisdictions if the front forks are drilled to take a front brake; in Britain this was the normal arrangement for a bike used for road time trials up to the 1960s or so.